This invention relates to an encoder of a multi-pulse type for use in encoding a speech signal into a plurality of excitation pulses.
A conventional encoder of the type described is revealed in U.S. application Ser. No. 153,290 filed Feb. 4, 1988, by Taguchi, namely, the instant applicant and assigned to the instant assignee. The encoder is used in general in combination with a decoder which is used as a counterpart of the encoder.
In the conventional encoder, the speech signal is divided into a sequence of frames. The speech signal is encoded into a plurality of excitation pulses for each frame by the use of a pulse search method known in the art. Each of the excitation pulses has an amplitude and a location determined by the speech signal. The encoder comprises a quantizer having a predetermined number of quantization levels and quantizes the excitation pulses into a quantized pulse signal. The encoder transmits the quantized pulse signal to the decoder through a transmission medium. If circumstances require, the quantized pulse signal is once memorized in a memory and then supplied to the decoder.
The decoder decodes the quantized pulse signal into a decoded signal and produces the decoded signal as a synthetic speech signal. Quality of the synthetic speech signal is influenced in general by the number of the excitation pulses and the number of the quantization levels or steps.
Generally speaking, when the speech signal represents voiced sound to have high electric power, the speech signal can be characterized by a small number of excitation pulses. The decoder can therefore produce a favorable synthetic speech signal regardless of the number of the excitation pulses. The decoder is, however, influenced by quantization noise. The encoder therefore must quantize the excitation pulses with a large number of quantization levels.
On the other hand, when the speech signal represents unvoiced sound to have low electric power, the speech signal must be characterized by a large number of excitation pulses. The decoder therefore requires the large number of excitation pulses in order to derive the favorable synthetic speech signal. The decoder is, however, not influenced by the quantization noise. The encoder therefore may quantizes the excitation pulses with a small number of quantization levels. The conventional encoder is, however, constant in number of the excitation pulses and the quantization levels regardless of the electric power. The decoder used as a counterpart of the conventional encoder is therefore restricted in quality of the synthetic speech signal.